1. Field of the Invention
The present invention relates to apparatus for determining the condition of a transformer and is concerned with apparatus for extracting and determining the volume and composition of gases dissolved or entrapped in transformer cooling oil.
2. Description of the Related Art
It is usual for relatively high power transformers, as are used in e.g. diesel electric locomotives and for stepping up or down the voltage in the high voltage electricity transmission grid, to include a volume of oil in cooling passages, through which the oil is generally pumped, in order to dissipate the heat generated by the transformer to the atmosphere. If such a transformer should develop a fault, such as a local short circuit, additional heat is generated at that point and this is transmitted to the oil in the vicinity of the fault. This additional heat results in thermal degradation of the oil and thus in the generation of gases which dissolve in the oil or remain entrapped in it in the form of small bubbles. The composition of these gases gives an indication of the temperature at the fault and thus of the nature of the fault and the volume of the gases provides an indication of the severity of the fault.
It is known to check the condition of a transformer by periodically removing a sample of the oil and determining the composition and volume of gas therein so as to detect the possible presence of a fault which can not be readily recognised by other means. For example, with reference to the apparatus of FIG. 1, this apparatus includes an oil receiver 2 which is connected to an oil inlet line 4 and to a dump tank 6 via an oil outlet line 8. The receiver 2 is also connected via a vacuum line 10 to a vacuum source 12, which is also connected to a pressure gauge 14, and via a gas outlet line 16 to a Toepler pump 18. A Toepler pump has only a single port and comprises a vessel containing mercury whose level within the vessel may be manually raised and lowered to expel or induce gas through the port. The port of the Toepler pump is connected via an outlet line 20 to a metering space or transparent burette 22 which contains visible volumetric markings and which also contains a volume of mercury which is in communication with further mercury in a mercury reservoir 24. The mercury within the burette 22 is also in communication with further mercury contained in an open-topped levelling vessel 26, whose function will be explained below. The top of the burette 22 also communicates via a line 28 with an analysing apparatus, typically a gas chromatograph 30. The various lines referred to above include a number of stopcocks or valves, whose purpose will be described below.
In use, the whole apparatus is evacuated by the vacuum pump 12 to a predetermined suitable vacuum by opening the valves 38,40,42,44,46,48 and 51. A sample of transformer cooling oil is tapped off into a sample bottle 32 and the oil inlet line 4 is introduced into the bottle and the valves 34 and 35 in the inlet line are opened briefly and then closed to flush the line 4. The valves 38 and 46 are closed and oil is drawn into the calibrated receiver 2 by opening valves 34 and 36 until it is visually determined to have reached a predetermined level. The valve 36 is then closed and the valve 48 is adjusted to connect the line 20 to the burette 22. The oil is stirred with a magnetically operated stirrer to aid removal of gas from the oil. The valves 44 and 46 are then closed and opened respectively and the mercury in the Toepler pump is raised to expel gas into the burette. The valves 44 and 46 are then opened and closed respectively and the mercury level in the Toepler pump is lowered to recreate a vacuum for further extraction of gas from the oil. This process is repeated a number of times until most of the gas in the oil has been transferred into the burette and the burette valve 48 is then closed. The height of the levelling vessel 26 is then adjusted manually until the levels of the mercury within it and the burette are the same which indicates that the gas within the burette is at atmospheric pressure. The volume of this gas is then read visually from the calibrations on the burette. The valve 51 is then closed and valve 48 opened to connect the burette 22 to the evacuated line 28 and the gas chromatograph 30. The height of the levelling vessel 26 is adjusted further to reassert atmospheric pressure within the line 28 and gas chromatograph 30. The gas chromatograph is then manually started to analyse the contained gas. The volume and composition of the gas within a known volume of transformer oil are thus determined and the condition of the transformer and an indication of the nature and severity of any fault in the transformer may be derived from these values.
Whilst the apparatus referred to above is adequate for its purpose, it suffers from two major disadvantages. Firstly, the operation of the Toepler pump, the determination of the gas volume and the operation of the numerous valves are all manual operations which means that the use of the apparatus is extremely laborious and time consuming and thus expensive. In practice, it is difficult for an operative to analyse more than about ten oil samples per day. Secondly, the apparatus includes a relatively large volume of mercury which represents a considerable potential hazard to the health of the operative and to the environment, particularly if breakage of any of the mercury-containing vessels should occur.
The object of the invention is therefore to provide apparatus for determining the volume and preferably also the composition of the gases in transformer oil which suffers from neither of the disadvantages referred to above.